The present invention relates to a vortex flow meter for measuring fluid flow, and more particularly, a flow meter for use with high pressure process fluid.
Various differential pressure sensitive vortex flow meters have been advanced, which operate on a principle that a buff body or shedding bar placed in a fluid flow causes or generates vortices alternately on opposite sides of the shedding bar, causing variations in pressure on either side of the bar. The frequency of vortex shedding for an individual bar configuration characteristic is directly proportional to the velocity of flow in the stream.
Vortex flow meters are known in the prior art, and examples of vortex flow meter implementations can be found in U.S. Pat. No. 4,926,695 issued to Kleven et al. on May 2, 1990, U.S. Pat. No. 5,343,762 issued to Beulke on Sep. 6, 1994, which are incorporated herein by reference.
Typically, a vortex flow meter for measuring fluid flow includes a conduit having a wall surrounding a bore for carrying the fluid. The wall has a wall region of reduced thickness formed therein. The wall region of reduced thickness is sometimes referred to as a “region of reduced stiffness” or a “flexure”. A shedding bar is disposed in the bore. In a typical embodiment, the shedding bar includes an upstream extremity, a downstream extremity and an intermediate portion connecting the upstream and downstream extremities. The intermediate portion includes a region of reduced stiffness which flexes in response to disturbances or vortices within the fluid created by fluid flow around the upstream extremity to promote motion of at least a portion of the downstream extremity. Such flow meters further include sensing means coupled to the downstream extremity for sensing the motion and providing an output as a function thereof. Generally, the sensing means senses lateral motion, and is removably attached to a post extending from the wall region away from the bore, wherein the post transmits the motion to the sensing means.
One method of assuring that a vortex meter meets process pressure retention (strength) requirements is described in the American Society of Mechanical Engineers (ASME) Boiler Pressure Vessel Code (BPVC). To determine the factor of safety the meter has at a given pressure, a meter representative of the design is pressurized until the structure bursts. Factors are derived from testing of the first meter body produced, and calculations are performed based on various factors of the material utilized to make the device. For example, material characteristics, including composition and manufacturing processes, are factored into a calculation to determine the maximum pressure to which a device may be rated.
For higher pressures, the flexure often presents a weak point in the structure, which can tear or burst open when exposed to higher pressures. Since the flexure must be designed to be thin enough to permit movement of the post that is coupled to the buff body or shedding bar, conventional flexures have difficulting providing the required safety factor at high pressures.